1. Field of the Invention
The present invention relates to a light source used for an optical communication, and more particularly to a wavelength division multiplexing light source applicable to an optical communication system.
2. Description of the Related Art
A wavelength division multiplexing passive optical network involves multiplexing of a plurality of optical signals having different wavelengths so that the multiplexed optical signals can be transmitted/received on a single optical fiber. The wavelength division multiplexing passive optical network comprises a central office for detecting upstream optical signals received from a plurality of subscribers so that communication services are provided to the subscribers, respectively, and remote nodes disposed between the central office and the respective subscribers for connecting the central office and the respective subscribers.
The central office modulates data to be provided to the respective subscribers in the form of optical signals and then outputs the modulated data. The central office creates downstream optical signals having different wavelengths for a subsequent transmission to respective subscribers. In this regard, the central office includes an optical source for creating the downstream optical signals.
A distributed feedback laser array, a multi-frequency laser, a spectrum-sliced light source, and an injection-locked Fabry-Perot laser with incoherent light may be used for creating the downstream optical signals. Further, a reflective semiconductor optical amplifier may be used for amplifying and modulating incoherent light inputted from the outside. The wavelength division multiplexing light source comprises a light source for outputting a broadband light, an optical element for dividing the light outputted from the light source into optical signals having different wavelengths, and external modulators for modulating the optical signals.
A light emitting diode, a super-luminescent diode, a Fabry-Perot laser, an optical fiber amplifier, and a ultra-short pulse laser may be used as light source. The light emitting diode and the super-luminescent diode have wide bandwidths and relatively inexpensive, and the Fabry-Perot laser provides high output at a nominal cost. As the optical element for spectrum-slicing the broadband light into optical signals having different wavelengths, an optical filter or a waveguide grating router (WGR) may be used. The external modulators are used for modulating and outputting the optical signals sliced from the optical element. LiNbO3 may be used as external modulator.
In the case of the super-luminescent diode and the light emitting diode, however, the bandwidths and outputs that can be modulated are low even though their output light bandwidths are wide. As a result, the super-luminescent diode or the light emitting diode is not a suitable choice as a light source for creating downstream optical signals outputted to the respective subscribers. Also, the Fabry-Perot laser has a narrow bandwidth. As a result, the Fabry-Perot laser is not also suitable as a wavelength division multiplexing light source requiring a broadband light. Furthermore, the Fabry-Perot laser has a drawback in that mode partition noise is generated between the optical signals since the Fabry-Perot laser modulates spectrum-sliced optical signals at a high speed and outputs the modulated optical signals, thus degrading the performance.
An injection-locked light source comprises a broadband light source for creating a broadband light with incoherent lights having different wavelengths, which are necessary to induce an injection-locked phenomenon, a demultiplexing element for demultiplexing the broadband light into the respective incoherent lights, and Fabry-Perot lasers for creating injection-locked downstream optical signals with the incoherent lights.
The Fabry-Perot laser creates injection-locked optical signals, which are obtained by directly modulating the incoherent lights. As such, no additional external modulator is required, and the wavelengths of the injection-locked optical signals can be controlled by changing the wavelengths of the incoherent lights. Thus, a plurality of optical signals having different wavelengths can be created by means of Fabry-Perot lasers of one kind by inputting incoherent lights having different wavelengths to the Fabry-Perot lasers, respectively.
Moreover, a reflective semiconductor optical amplifier has operational characteristics similar to those of the injection-locked light source. In particular, a broadband light created at the broadband light source is demultiplexed into incoherent lights, and then the corresponding demultiplexed incoherent light is modulated and amplified.
With the injection-locked Fabry-Perot laser, however, an injection-locked phenomenon is generated only when the intensity of the inputted incoherent light exceeds a prescribed intensity value. Consequently, incoherent light having a high-output intensity is required. The transmission property of the reflective semiconductor optical amplifier is improved by means of gain saturation generated when the intensity of incoherent light inputted thereinto is large. Consequently, a high-output broadband light source is required. Therefore, there is a need for an improved optical transmitter at a low cost.